[go_router] canPop() throws "BadState: No element" on error pages (and maybe other pages)

[quaaantumdev]

Steps to reproduce

1. Implement errorBuilder for GoRouter with some CustomErrorPage() widget.
2. Check if you can pop the page inside the CustomErrorPage() by accessing GoRouter.of(context).canPop() inside the build(..) function of your CustomErrorPage
3. Navigate to any URL that does not exist (which should bring up the error page on an empty route match list)

Expected results

Expecting GoRouter.of(context).canPop() to return false if there is not even a single page to pop.

Actual results

Accessing canPop() will crash with Bad state: No element .

One may also trigger this error in other ways, that is just the way I discovered it. I wanted to use my default Header on my error page, which internally checks if it should display a back button, resulting in this error. I guess this is a pretty common use case.

The Problem seems to be, that GoRouter.canPop delegates it to routerDelegate.canPop() which then tries to get the "last" match by accessing currentConfiguration.matches.last. This throws as there are no matches. The implementation of Navigator.canPop() returns false if there is no route, so i guess it should be changed to currentConfiguration.matches.lastOrNull and return false if no route was found. But whatever value it may return from your point of view, throwing (by accident, i'd say) seems really wrong.

Code sample

Initialize GoRouter

// MaterialApp.router(...
GoRouter(
  // ... your config
   errorBuilder: (context, state) => CustomErrorPage(
        routerState: state,
   ),
)

/// a custom error page
class CustomErrorPage extends StatelessWidget {
  final GoRouterState routerState;

  const GoErrorPage({
    super.key,
    required this.routerState,
  });

  @override
  Widget build(BuildContext context) {
    final canPop = GoRouter.of(context).canPop(); // <-- this will crash
    // show a back button if one can pop ...
    return Text("Hello there");
  }
}

Screenshots or Video

No response

Logs

No response

Flutter Doctor output

Doctor output

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[darshankawar]

@quaaantumdev
Can you provide complete runnable code sample that we can directly copy paste and run to verify this further ?
Also provide the entire error log.

[quaaantumdev]

@darshankawar we created a sample app showing the issue: https://github.com/quaaantumdev/gorouter_canpop_issue

HomePage works as expected:

Pressing the button tries to navigate to a route that does not exist, resulting in the CustomErrorPage. This Error Page utilizes the same CustomPageHeaderSliver, which checks GoRouter.of(context.canPop(), which will therefore fail. We catch this exception in the example to show it in the ui:

And here is your error log from the CustomErrorPage --> CustomPageHeaderSliver.build() --> GoRouter.of(context).canPop()

Bad state: No element
#0      _Array.last (dart:core-patch/array.dart:56:5)
#1      GoRouterDelegate.canPop (package:go_router/src/delegate.dart:89:58)
#2      GoRouter.canPop (package:go_router/src/router.dart:321:35)
#3      CustomPageHeaderSliver.build (package:gorouter_canpop_issue/main.dart:115:37)
#4      StatelessElement.build (package:flutter/src/widgets/framework.dart:5550:49)
#5      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5480:15)
#6      Element.rebuild (package:flutter/src/widgets/framework.dart:5196:7)
#7      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:5462:5)
#8      ComponentElement.mount (package:flutter/src/widgets/framework.dart:5456:5)
#9      Element.inflateWidget (package:flutter/src/widgets/framework.dart:4335:16)
#10     MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6893:36)
#11     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/fra

[darshankawar]

Thanks for the update. Seeing the same behavior as reported using the code repo provided.

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